Fix #2444: Ambiguous implicit conversions from null literals in decompiled code

ICSharpCode.Decompiler.Tests/TestCases/Correctness/OverloadResolution.cs

@@ -36,6 +36,7 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Correctness
 			Issue1747();
 			CallAmbiguousOutParam();
 			CallWithInParam();
+			Issue2444.M2();
 		}
 
 		#region ConstructorTest
@@ -334,6 +335,38 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Correctness
 		}
 #endif
 		#endregion
+
+		#region #2444
+		public struct Issue2444
+		{
+			public class X { }
+			public class Y { }
+
+			public static implicit operator Issue2444(X x)
+			{
+				Console.WriteLine("#2444: op_Implicit(X)");
+				return new Issue2444();
+			}
+
+			public static implicit operator Issue2444(Y y)
+			{
+				Console.WriteLine("#2444: op_Implicit(Y)");
+				return new Issue2444();
+			}
+
+			public static void M1(Issue2444 z)
+			{
+				Console.WriteLine(string.Format("#2444: M1({0})", z));
+			}
+
+			public static void M2()
+			{
+				Console.WriteLine("#2444: before M1");
+				M1((X)null);
+				Console.WriteLine("#2444: after M1");
+			}
+		}
+		#endregion
 	}
 
 	class IndexerTests

ICSharpCode.Decompiler/CSharp/CallBuilder.cs

@@ -1148,7 +1148,7 @@ namespace ICSharpCode.Decompiler.CSharp
 			var conversions = CSharpConversions.Get(expressionBuilder.compilation);
 			IType targetType = method.ReturnType;
 			var conv = conversions.ImplicitConversion(argument.Type, targetType);
-			if (!(conv.IsUserDefined && conv.Method.Equals(method)))
+			if (!(conv.IsUserDefined && conv.IsValid && conv.Method.Equals(method)))
 			{
 				// implicit conversion to targetType isn't directly possible, so first insert a cast to the argument type
 				argument = argument.ConvertTo(method.Parameters[0].Type, expressionBuilder);